TECTONIC EVOLUTION OF WALVIS RIDGE, OFFSHORE NAMIBIA: EVIDENCE FROM REFLECTION SEISMIC AND HIGH-RESOLUTION BATHYMETRIC DATA

Expeditions 17/1 and 17/2 of R/V Maria S. Merian, carried out geophysical surveys between November 2010 and February 2011 in the area around Walvis Ridge, Southeast Atlantic Ocean. Among the data collected, reflection seismic and high-resolution bathymetric data offer some important preliminary insights into the tectonic evolution of the ridge and the adjoining lower continental slopes and ocean basins.

The NE-SW trending Walvis Ridge has a trapezoid shape and is likely built up by thick sequences of plateau basalts, with top of basement inclined to the south. Sediments are almost absent on the NW side of the ridge, preserving a fascinating mountainscape formed early in the tectonic history. Sediment cover grows southward to thicknesses up to 2 s TWT, above a basement characterized by dipping reflector sequences inclined NW. There is progressive northward onlap of sedimentary strata, indicating that in the earlier part of its history parts of Walvis Ridge may have formed a land surface. This is further supported by denudational features, like steep cliffs up to 150 m high, and deeply incised valleys, defining paleo-drainages. Isolated, flat-topped guyots seaward of the ocean-continent boundary attest to a later history of wave abrasion and progressive subsidence of Walvis Ridge.

The boundary zone towards the Angola Basin to the north is an important extensional or transtensional tectonic feature, conincident with the Florianopolis Fracture Zone on the Brazilian side of the Atlantic. The fault zone has recorded two important episodes of vertical differential movement, documented by thick half-graben fills and clastic sediment aprons on the Angola Basin crust. The early one is likely connected to continental break-up. The later one probably occurred in the Middle Tertiary, also affecting the SE side of Walvis Ridge. We conclude by stating that the record of sedimentary and geomorphologic processes around Walvis Ridge can contribute important information to decipher the tectonic history of the southeast Atlantic.